1. Field of the Invention
This invention relates to an aqueous ink, an organic pigment powder, an ink cartridge, a recording unit and an ink-jet recording apparatus.
2. Related Background Art
While dyes are mainly used as coloring materials of inks for ink-jet recording, massive research efforts have been made for developing inks using pigments as coloring materials because images formed by dye inks are not highly water resistant nor light resistant. Particularly, in recent years, ink-jet recording apparatus adapted to use inks containing color pigments have been marketed as so-called wide format printers to be used for various industrial applications including printing posters, panels, signs and pop advertisements. The recent technological development in the field of manufacturing pigment ink has triggered various technological proposals for methods of dispersing pigments that are good for ink-jet recording in aqueous solvents as well as methods of manufacturing such pigments.
As a result, problems concerning pigment ink for ink-jet recording including clogged nozzles of ink-jet recording heads and lack of long term storage stability have been alleviated if compared with the initial stages of development.
On the other hand, the color developing effect of ink for images printed by ink-jet recording has been steadily improved as a result of the development in recent years of optimal ink receiving layers arranged in recording mediums that has been made concurrently with the development of pigment ink, although such recording mediums are exclusively designed for particular types of ink-jet recording apparatus.
Additionally, inks containing pigment particles which are finely divided or have hydrophilic properties in order to stably disperse such pigment particles have been proposed in an effort for improving the quality of images formed by ink-jet printing using pigment ink. Japanese Patent Application Laid-open No. 11-49974 discloses a granular organic pigment showing an improved dispersibility obtained by introducing sulfonic acid group in a liquid phase and aqueous ink containing such a granular pigment. Japanese Patent Application Laid-open No. 11-57458 and Japanese Patent Application Laid-open No. 11-333288 disclose a method of manufacturing ultra-fine particles of an organic pigment by using a vapor-phase method and reforming the surface of the particles by means of a plasma treatment.
As a result of research efforts, the inventors of the present invention found that an organic pigment containing anthraquinone as main ingredient is suitably micro-granulated by means of a vapor-phase method to form ultra-fine organic pigment powder and that highly stable aqueous ink can be obtained by using such an organic pigment powder.
It is an object of the present invention to provide aqueous pigment ink that can be used to form high quality images.
Another object of the present invention is to provide very fine organic pigment powder that is excellent in terms of color tone and stability.
Still another object of the present invention is to provide an ink-jet recording apparatus that forms high quality images as well as an ink cartridge and a recording unit.
According to the invention, the above object is achieved by providing an aqueous ink comprising an organic pigment micro-granulated by means of a vapor phase method, the pigment containing anthraquinone as principal ingredient, wherein the organic pigment has a number average particle diameter of primary particles of 10 to 50 nm.
In another aspect of the invention, there is provided organic pigment powder containing anthraquinone as principal ingredient, the powder being micro-granulated by means of a vapor phase method, wherein the organic pigment powder has a number average particle diameter of primary particles of 10 to 50 nm and has a hydrophilic group introduced by means of a plasma treatment.
In still another aspect of the invention, there is provided an ink cartridge containing an aqueous ink according to the invention.
In still another object of the invention, there is provided a recording unit comprising:
an ink container containing an aqueous ink for ink-jet operation, the ink comprising an organic pigment micro-granulated by means of a vapor phase method, the organic pigment containing anthraquinone as principal ingredient, wherein the organic pigment has a number average particle diameter of primary particles of 10 to 50 nm; and
an ink-jet recording head for ejecting ink from the ink container.
In a further aspect of the invention, there is provided an ink-jet recording apparatus comprising:
an ink container containing an aqueous ink for ink-jet operation, the ink comprising an organic pigment micro-granulated by means of a vapor phase method, the organic pigment containing anthraquinone as principal ingredient, wherein the organic pigment has a number average particle diameter of primary particles of 10 to 50 nm; and
an ink-jet recording head for ejecting ink from the ink container.
Thus, according to the present invention, there is provided pigment ink containing very fine organic pigment powder and adapted to form images with an enhanced level of transparency comparable to images formed by dye ink. According to the invention, there is also provided very fine organic pigment powder that can be dispersed stably in an aqueous medium. While the reason why an organic pigment containing anthraquinone as principal ingredient can be micro-granulated by means of a vapor phase method without damaging its color tone is not known, the inventors of the present invention presume that, while azo-type pigments are apt to be decomposed in the course of micro-granulation by means of a vapor phase method, anthraquinone-type organic pigments are not because they are highly thermally stable. As a side effect of micro-granulation of an organic pigment by means of a vapor phase method, the content of polyvalent metal ions and that of silicon atoms in the pigment can be reduced from those of the pigment before micro-granulation. While these elements contained in the ink may not adversely influence the performance of the ink and may rather be effective for ink-jet recording operations, the polyvalent metal ions and the silicon atoms contained in a pigment are impurities mainly introduced during the pigment manufacturing process and their contents may vary from lot to lot so that the concentration of these elements preferably may well be minimized. According to the invention, the concentration of these impurities can be reduced to effectively improve the uniformity of ink quality.